This invention relates to a filtration device equipped with a plurality of filter elements having different filtering characteristics. More particularly but not exclusively, it relates to a filtration device for removing particulate matter and odors from air, such as the air in a cabin of an aircraft.
Filters for use in certain working environments need to be designed for minimum total pressure drop across the filter. This is particularly true in the case of filters for use in aircraft cabins. A low total pressure drop across such a filter is desirable because the lower the total pressure drop, the smaller are the size and the weight of the air flow equipment, such as blowers, necessary for passing air through the filter, permitting a decrease in the weight of the aircraft or an increase in the cargo that can be carried.
The air in an aircraft cabin contains a large variety of contaminants which it is desirable to remove, such as fibers from clothing, carpets, and seat cushions, cigarette smoke, odors from the galley and lavatories, and human body odors. Fibers and a certain amount of the tar in cigarette smoke can be effectively removed by a particulate filter, which employs a mass of fibers to entrap particulate contaminants. Minute aerosol particles and odors, on the other hand, are usually removed from air or rendered innocuous by what is referred to as a sorbent filter. A sorbent filter normally employs an adsorbent, such as activated charcoal, which adsorbs odor-causing hydrocarbons and other chemicals from air passing through the filter. The air which leaves the sorbent filter has a lower concentration of the substances responsible for the odors, so the air is less odorous.
In an aircraft it is generally desirable to install a particulate filter and a sorbent filter in series in close proximity. One reason for this manner of installation is to save space inside the aircraft, which is always at a premium. Another reason is that a sorbent filter usually does not have a large external surface area, so unless the air entering the sorbent filter is free of fibers, the external surface of the sorbent filter can soon become clogged with fibers, resulting in a high pressure drop across the filter. Therefore, it is desirable to install a particulate filter just upstream of a sorbent filter. However, because the airflow characteristics are quite different for a particulate filter and a sorbent filter, conventional arrangements have not resulted in a minimal total pressure drop and have not made optimal use of space.